Bump master to support 21

Signed-off-by: Julius Härtl <jus@bitgrid.net>

appinfo/info.xml

@@ -36,7 +36,7 @@
         <database min-version="9.4">pgsql</database>
         <database>sqlite</database>
         <database min-version="5.5">mysql</database>
-        <nextcloud min-version="18" max-version="20" />
+        <nextcloud min-version="18" max-version="21" />
     </dependencies>
     <background-jobs>
         <job>OCA\Deck\Cron\DeleteCron</job>